1. Field of the Invention
This invention relates to electrical connectors for infrared detectors and, more particularly, to arrangements for improving the reliability of connections to a plurality of sensors in a detector array assembly which is subject to thermal fatigue from temperature cycling.
2. Description of the Related Art
In the present fabrication of focal plane arrays for infrared sensing systems, the hybrid detector array assembly comprises a pair of microchips, one bearing the array of sensors and the other bearing a corresponding array of cells or diodes with associated contact pads to provide the readout of individual sensor signals. The contact pairs of the two microchips are joined together in a process called hybridization. In this process, a plurality of indium bumps on the detector chip and a corresponding plurality of indium bumps on the readout chip are cold welded together by pressure Once joined, they are no longer separable and the breaking of any weld constitutes a failure of that readout cell.
Over time an infrared detector array is repeatedly cycled between room temperature and its normal operating temperature of 77 degrees K. This repeated temperature cycling is responsible for problems relating to thermal fatigue which results from the different coefficients of thermal expansion in the different materials present in the hybrid detector assembly.
In the present (prior art) fabrication process, the indium bumps are made by vapor deposition through a photo-reduced mask pattern and have a typical height of 6-9 microns. It is not possible to deposit the indium bumps more than 10 microns high with acceptable quality and density. Over the temperature cycling range between room temperature and 77 degree K. operating temperature, the various materials present in the array account for the thermal fatigue problems. For example, the readout chip is a silicon substrate with contact pads approximately 0.00016 centimeter square on 0.0008 centimeter inch spacing. A typical array may have 128.times.128 cells. The sensors are arranged in a similar array on a cadmium telluride substrate. Because of the differences in thermal expansion and contraction between the detector chip and readout chip, repeated temperature cycling results in various failure modes: contact pads are pulled away from the substrate, pieces of contacts break off, the cold welded junctions of the indium bumps fracture and separate, the stresses induced by the differential thermal expansion or contraction of the substrates may cause warpage of the array chips, and the like.
Arrangements in accordance with the present invention incorporate a particular material known as a shape memory alloy in a novel arrangement to overcome some of the problems described hereinabove. Shape memory alloys are a unique family of metals which exhibit a temperature dependent shape change. They can be deformed from 5 to 8 percent in tension, compression or shear. Upon heating beyond a critical temperature, the metal returns to its original "memory" shape and, if resisted, can generate stresses as high as 100 kpsi. Stresses, strains, transition temperatures and other parameters of such materials can be controlled by composition and processing to tailor the material to provide particular performance characteristics in a given application.
This unusual effect of shape memory depends upon the occurrence of a specific type cf phase change known as martensitic transformation. Martensite forms on cooling from the high temperature phase, termed austenite, by a shear type of process. The curves of deformation with temperature and stress exhibit a hysteresis effect. Shape memory alloy products have been produced by Raychem Corporation, Menlo Park, Calif. The materials of interest here are sold by Raychem under the trademark Tinel.